1. Field of the Invention
The present invention relates to color proofing films. More particularly, the present invention pertains to a negative working, peel-apart photosensitive element capable of producing negative images upon imagewise exposure to actinic radiation and subsequent peel-apart development. Such color proofing films produce multicolored negative images on a receiver sheet by successive imagewise exposures to actinic radiation and peel developments.
2. Description of the Prior Art
In the field of lithographic printing, it is desirable to produce a multicolor proof to assist in correcting a set of color separation films prior to using them to produce metal based lithographic printing plates. The proof should reproduce the color quality that will ultimately be obtained during the printing process. The proof must be a consistent duplicate of the desired halftone image. Visual examination of a color proof should show the color rendition to be expected from a press printing using the color separations, and any defects on the separations which might need to be altered before making the printing plates.
It is known in the art to produce color proofs for multicolor printing by using a printing press or proof press. However, this procedure requires that all of the actual printing steps be performed including making expensive metal printing plates. As a result, this conventional method of color proofing is costly and time consuming.
Photoimaging processes can also be used to produce a color proof. There are two general types of photoimaging methods, namely the overlay type and the single sheet type. In the overlay type of color proofing method, an independent transparent plastic support is used for producing an image of each color separation film by applying a photosensitive solution of the corresponding color. A series of such supports carrying images of the corresponding colors are then superimposed upon each other over a white sheet to produce a color proofing composite. The primary advantage of the overlay method is that proofs can be made quickly and can serve as a progressive proof by combining any two or three colors in register. However, this type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet. As a result, the impression of the color proofing composite thus prepared is vastly different from that of copies actually obtained with conventional printing presses and with proof presses. Examples of such overlay approaches are contained in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films on a single receiver sheet. This can be accomplished by sequentially applying colorants or colored, photosensitive layers to a single opaque support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system. Examples of such single sheet approaches are contained in U.S. Pat. Nos. 3,574,049; 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642.
Peel apart color proofing systems are also well known. U.S. Pat. Nos. 4,963,462; 5,049,476; 4,910,120 and 5,108,868 disclose peel developable, single sheet color proofing systems. U.S. Pat. No. 4,489,154, discloses a process which produces a single layer color proof by peel development. The photosensitive material comprises a strippable cover sheet; a colored photoadherent layer; a nonphotosensitive organic contiguous layer; and a sheet support. The material is exposed and peel developed. The positive or negative image is transferred to a receiver base. A fresh layer of adhesive must be applied to the receptor for each subsequent transfer.
It has been a problem in the art to produce color proofing images having a high degree of resolution. In this regard, U.S. Pat. No. 5,300,399 discloses one type of peel apart color proofing system to produce a negative-acting color proofing film. This element sequentially comprises a strippable cover sheet which is transparent to actinic radiation; a crosslinked release layer; a color layer; a photoadhering layer; a thermoplastic adhesive layer; and a receiver sheet. At least one of the color layer and the photoadhering layer contains a photoinitiator. A single sheet, negative working color proofing film having good image quality with high resolution is produced when exposed through the strippable cover sheet. The cover sheet is subsequently peeled away, removing the unexposed areas of the color layer and transferring the exposed areas of the color layer to the receiver sheet.
U.S. Pat. No. 5,686,221 teaches another proof-making process which requires contacting a photomask with the clear adhesive layer of the first, unlaminated, color film; exposing the film imagewise to actinic light through a photomask and the adhesive layer; laminating the exposed film to a temporary receiver with the adhesive layer facing the receiver; peeling away the clear PET support from the temporary receiver, with the exposed areas of the color layer remaining attached to the temporary receiver, optionally repeating steps 1-4 for another color film in register; laminating the temporary receiver bearing a multicolor image on it to a permanent receiver and peeling them apart, thus transferring the multicolor image to the permanent receiver. It has now been unexpectedly found that images with improved resolution compared to those produced by U.S. Pat. No. 5,300,399 can be obtained via a process different and simpler than the one taught by U.S. Pat. No. 5,686,221.
The method of this invention improves on the resolution obtainable by U.S. Pat. No. 5,300,399 by first imagewise exposing the unlaminated photosensitive element and then laminating it to its receiver and peel developing. While a typical resolution obtained by U.S. Pat. No. 5,300,399 is 10-95% dots on a 150 l/inch screen ruling, a typical resolution obtained by this invention is 2-97% dots on a 150 l/inch screen ruling. The inventive method improves on U.S. Pat. No. 5,686,221 by eliminating the need for a temporary receiver and extra lamination step.